Bioorthogonal reactions are reactions of materials with each other, wherein each material has limited or substantially no reactivity with functional groups found in vivo. The efficient reaction between an azide and a terminal alkyne, i.e., the most widely studied example of “click” chemistry, is known as a useful example of a bioorthogonal reaction. In particular, the Cu(I) catalyzed 1,3-dipolar cyclization of azides with terminal alkynes to give stable triazoles (e.g., Binder et al., Macromol. Rapid Commun. 2008, 29:952-981) has been employed for tagging a variety of biomolecules including proteins, nucleic acids, lipids, and saccharides. The cycloaddition has also been used for activity-based protein profiling, monitoring of enzyme activity, and the chemical synthesis of microarrays and small molecule libraries.
An attractive approach for installing azides into biomolecules is based on metabolic labeling whereby an azide containing biosynthetic precursor is incorporated into biomolecules using the cells' biosynthetic machinery. This approach has been employed for tagging proteins, glycans, and lipids of living systems with a variety of reactive probes. These probes can facilitate the mapping of saccharide-selective glycoproteins and identify glycosylation sites. Alkyne probes have also been used for cell surface imaging of azide-modified bio-molecules and a particularly attractive approach involves the generation of a fluorescent probe from a non-fluorescent precursor by a [3+2]cycloaddition.
Despite the apparent utility of reacting an azide with a terminal alkyne, applications in biological systems using this reaction have been practically limited by factors including the undesirable presence of a copper catalyst. Thus, there is a continuing, unmet need for new bioorthogonal reactions.